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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 8, Num. 4, 2000, pp. 419-428
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African Crop Science Journal, Vol. 8. No. 4, pp. 419-428
African Crop Science Journal, Vol. 8. No. 4, pp. 419-428
Physical, Chemical, and Water Absorption Characteristics
of Tropical Maize Hybrids
B.B. Maziya-Dixon, J.G. Kling and A.E. Okoruwa International Institute of Tropical Agriculture, c/o L.W. Lambourn & Co., Carolyn House, 26 Dingwall Road, Croydon CR9 3EE, England
(Received 4 February, 2000; accepted 13 June, 2000)
Code Number: CS00044
INTRODUCTION
The International Institute of Tropical Agriculture (IITA)
initiated a maize hybrid breeding programme in 1979 with support from the Nigerian
government (IITA, 1992). Release of the first hybrids in the early 1980s enabled
the establishment of several private seed companies in Nigeria. A strong private
seed sector can ensure a reliable supply of high-quality seed to farmers and
serve as a catalyst for agricultural development (Smith et al., 1997).
Hybrids developed at IITA were selected for their high grain yield, disease
resistance and standability, with relatively little emphasis on post-harvest
quality. Despite the superior agronomic performance of these hybrids, farmers
still prefer their traditional cultivars in some areas, because they possess
desired storage and processing characteristics for local maize dishes (Kling,
1993).
An understanding of the factors contributing to maize quality
for various end-uses is of importance in plant-breeding programmes aimed at
increasing acceptance of genetically improved food crops by farmers, consumers
and food processors (Stroshine et al., 1986; Peplinski et al.,
1989). Hybrids that produce shelled maize which dries rapidly, high in storage
mould resistance and dry milling quality would offer advantages to producers,
merchants, and end-users. Maize dry millers seek to optimise yield of large
grits and would benefit from the development or identification of hybrids, which
yield a greater percentage of flaking grits. Intrinsic quality characteristics
such as starch, oil, and protein content can be directly related to end-use
value (Hurburgh, 1989), and the potential for improving these characteristics
through genetic manipulation is quite high (OTA, 1989). Other maize characteristics
that have been found to affect yield and quality of maize products include test
weight, kernel density and size, kernel hardness, water absorptivity, and average
kernel weight (Paulsen and Hill, 1985; Pomeranz et al., 1986b).
The purpose of this study was to determine the physical, chemical,
and water absorption characte-ristics of tropical maize hybrids developed at
IITA as indicators of their suitability for primary processing in West and Central
Africa.
MATERIALS AND METHODS
Maize hybrids. Eleven maize hybrids that were included
in IITA International Trials in 1989 were selected for this study to represent
the range of available hybrids adapted to the lowland tropics of West and Central
Africa. Hybrids were grown at the IITA research farm, Ibadan, Nigeria (7°26N,
3°54E) under uniform conditions in 1990 during the first rainy season (March
to August) to standar-dise possible environmental effects on grain quality.
Within each hybrid plot, controlled pollinations were made to avoid pollen contami-nation
from other hybrids. At maturity, the maize was hand harvested and ears with
poor seed sets were discarded. Ears were artificially dried at 40°C in a
dryer. The final moisture content ranged from 11 to 12%. The ears were hand
shelled to minimise physical damage and were stored at 10°C and brought
up to 25°C before analyses. Kernel colour and texture of the hybrids studied
are presented in Table 1. The hybrids are of intermediate maturity (approximately
110 days to maturity in Ibadan).
TABLE 1.Characteristics of maize hybrids |
Hybrid |
Kernel colour |
Kernel texture |
8321-18 |
White |
Semi-flint |
8321-21 |
White |
Dent |
8338-1 |
White |
Dent |
8425-8 |
Yellow |
Dent |
8505-5 |
White |
Dent/flint |
8522-2 |
Yellow |
Flint |
8644-27 |
Yellow |
Flint |
8644-31 |
Yellow |
Dent |
8644-32 |
Yellow |
Dent/flint |
8705-4 |
White |
Semi-flint |
8705-6 |
White |
Semi-flint |
Physical analyses. 1000-kernel weight was determined
by counting and weighing 100 maize kernels and expressing the result as g/1000
kernels (Hilliard and Daynard, 1974). Kernel size (dimensions) was measured
by selecting 10 kernels at random and measuring the major axes (length, width,
and depth) with a vernier calliper (Martinez-Herrera and LaChance, 1979). Percent
kernel components parts expressed on dry matter basis (endosperm, germ, and
pericarp) were determined according to the method of Earle et al. (1946).
For the floaters test, the method of kernel density separation
as described by Wischer (1961) was followed with slight modification. A sodium
nitrate solution with specific gravity of 1.250 at ambient temperature was used
to measure the percentage of floating kernels (50 kernels in 500 ml of solu-tion).
The specific gravity of the solution was checked with a hydrometer before, during,
and after measurement.
Test weight (bulk density) was determined by weighing the amount
of grain held in a full to level calibrated 50 ml beaker based on a modification
of the method used by Kikuchi et al. (1982). An average of 10 measurements
represented one determination. Test weight was expressed as weight per unit
volume and values were converted to kg m-3.
The Stenvert Hardness Test (SHT) as described by Pomeranz et
al. (1985) was performed on 20 g maize samples. Samples were ground in a
micro hammer-cutter mill Type V (Glen Mills, Maywood, NJ), using a 2.0 mm aperture
screen and a hammer speed of 3,600 rpm. The time to grind 17 ml of meal, measured
to the nearest 0.1 sec, was used as a measure of grain hardness. Kernel density
was measured by placing 25 pre-weighed kernels in a graduated cylinder containing
20 ml of 50% ethanol (Arnold et al., 1977).
Water absorption capacity was determined by soaking 25 g in
100 ml of distilled water at room temperature. After 6 hrs, the surfaces of
the kernels were blotted dry, and the weight increase due to water absorbed
by the kernels was measured. The increase in weight expressed as percentage
of initial weight was calculated as the absorption index (Andah, 1977). All
physical measurements were done in triplicates. Results were averaged and reported
on a dry weight basis.
Chemical analyses. Fat content was determined by the
Soxhlet method (AOAC, 1984). Ash and moisture content of ground samples and
whole grains, respectively, were determined by AACC (1983) methods. Crude protein
(N x 6.25) was calculated based on Kjeldahl Nitrogen (N) determined by a standard
procedure using the Tecator Kjeltec System 1 (Tecator, 1979). Crude fibre was
determined by the trichloroacetic acid (TCA) method of Entwistle and Hunter
(1949).
The method of McCready et al. (1950) was used for the
extraction of soluble sugars. Sugar content was determined by the method of
Dubois et al. (1956). The residue from sugar analysis was used for determination
of starch content. For amylose determination, starch was extracted by the method
of Watson (1964). Amylose content of starch was determined with an Autoanalyzer
(Model AA II, Technicon Inst.) using the method of Juliano (1971). Amylopectin
content was obtained by subtracting the amylose content from the starch content
of the sample. The water binding capacity of starch was determined according
to the method of Medcalf and Gilles (1965). All chemical analyses were done
in duplicate, averaged, and reported on a dry weight basis.
Statistical procedures. Data were analysed using the
Statistical Analysis System (SAS) (SAS, 1985) using one-way analysis of variance
with the Fishers Protected Least Significant Difference (LSD) test for statistical
significant differences among means (Ott, 1988). The pooled variance among samples
within hybrids was used as the error term. Means were grouped by the LSD test
at the 5% level.
RESULTS AND DISCUSSION
Physical properties. Test weight, 1000-kernel weight,
kernel density, percent floaters, hardness index, and water absorption capacity
of maize samples is presented in Table 2. Among the hybrids examined, 8644-31
and 8425-8 had highest test weights while 8644-27 and 8338-1 had the lowest.
All other hybrids were intermediate. Maize with low test weight often has a
lower percentage of hard endosperm, and, therefore, produces a lower percentage
yield of prime, large grits when milled. Paulsen and Hill (1985) showed that
yield of flaking grits was significantly increased by selecting maize with high
test weight and low breakage susceptibility. Therefore, hybrids 8644-31 and
8425-8 would be more desirable for industrial dry milling based on our test
weight results.
Table 2. Means for selected physical characteristics
of maize hybridsa |
Hybrid |
Test weight (kg m-3) |
100-kernel weight (g) |
Kernel density (gm l-3) |
Percent floaters |
Hardness index (sec) |
Percent water absorption |
8321-18 |
764.7de |
293de |
1.30a |
0.0f |
39.8f |
32.7a |
8321-21 |
769.0cd |
290ef |
1.27ab |
16.0a |
43.3d |
20.8fg |
8338-1 |
753.3f |
310bc |
1.26abc |
10.7bc |
42.9cd |
22.7e |
8425-8 |
787.7a |
254g |
1.18d |
13.3ab |
39.8f |
24.6d |
8505-5 |
755.3bc |
315bc |
1.32a |
8.0cd |
41.2e |
22.0ef |
8522-2 |
760.3ef |
278f |
1.28ab |
4.0e |
40.3f |
29.8b |
8644-27 |
754.0f |
319b |
1.28ab |
10.7bc |
43.3c |
22.3e |
8644-31 |
789.7a |
313bc |
1.19cd |
6.7de |
47.9a |
19.7g |
8644-32 |
761.7e |
297de |
1.21bcd |
6.7de |
43.7c |
28.4c |
8705-4 |
775.0bc |
304cd |
1.26ab |
4.0e |
45.4b |
19.8g |
8705-6 |
780.3b |
352a |
1.21bcd |
9.3cd |
38.2g |
24.7d |
Mean |
770.1 |
302 |
1.25 |
8.12 |
42.26 |
24.3 |
LSD (0.05) |
7.07 |
12.0 |
0.08 |
3.54 |
0.89 |
1.23 |
C.V. |
0.54 |
2.35 |
3.56 |
25.72 |
1.24 |
2.99 |
aMeans within a column followed by the same letter
are not significantly different (P<0.05) |
Thousand kernel weight varied significantly among hybrids.
Hybrid 8705-6 had the highest kernel weight while 8425-8 had the lowest kernel
weight (Table 2). Both density of kernels and packing in the container influence
test weight measurements. According to Thompson and Isaacs (1967), maize has
an average void volume (space between kernels) of 42%. Therefore, measure-ments
that eliminate void spaces such as thousand kernel weight give a more accurate
volume measurement for density calculations. This might explain why Hybrid 8425-8
was highest for test weight and lowest for thousand kernel weight.
Kernel density varied greatly among the hybrids and ranged
from 1.18 to 1.30 g ml-3 indicating a fairly large spread in endosperm
types, from soft to hard. Although hybrid 8425-8 had higher test weight, it
had the lowest kernel density, 1000 kernel weight and high percent floaters
(Table 2). Kirleis and Stroshine (1990) found that maize density was the best
single predictor of dry milling quality and that a prediction model combining
test weight and kernel density improved the prediction of milling quality of
three dent hybrids. According to Watson (1987), differences in chemical composition
among individual kernels of dent maize are probably too small to account for
the wide differences in density observed. However, kernels do differ in the
amount of void space within them and in the ratios of corneous to floury endosperm.
Corneous endosperm is very dense, whereas floury endosperm is full of microfissures
or void spaces.
Significant differences were observed for percentage floaters
(Table 2). Among the hybrids, 8321-21 had the highest percentage floaters while
8321-18 had no floaters. In general, the samples were classified as either dent,
semi-flint, or flint. This could explain the low range of percentage floaters
(0-16%) observed in this study. The floaters test indicates the amount of corneous
(hard) endosperm in the kernel. Hybrids with low percentage floaters would be
expected to be harder than those with high percentage floaters, and, therefore,
will be suitable for dry milling and for production of rice-like products.
Based on the Stenvert Hardness Index, four Hybrids (8321-18,
8522-2, 8425-8, 8705-6) were significantly softer than others. Hybrids 8705-4
and 8644-31 exhibited a significantly greater hardness among the Hybrids tested
(Table 2). Hardness is an intrinsic characteristics that can be altered by genetics
and environments. It is also closely related to the degree of adhesion between
starch and protein. Hardness and softness are milling characteristics relating
to the way the endosperm breaks during milling. The ratio of dense corneous
endosperm to floury endosperm causes variation in kernel hardness. Maize with
a higher proportion of corneous endosperm is typically rated harder by mechanical
measures of hardness. In a study conducted by Pomeranz and Czuchajowska (1987),
high yields were obtained from maize with the highest test weight and hardness
index, as determined by percent of coarse fraction from the hardness tester.
Pomeranz et al. (1986a) evaluated ten different commercial maize hybrids
for relative hardness using several different methods of hardness evaluation.
Stenvert Hardness values ranged from 20.4 sec (hard) to 12.0 sec (soft). In
the present study, values ranged from 38.2 sec to 47.9 sec. Selection for resistance
to ear rot and ear borers at IITA may have favoured selection of flintier grain
type in comparison to hybrids developed in temperate areas where disease and
insect pressures are less severe.
For the water absorption index of grain, 8321-18 had significantly
higher value than 8644-31and 8705-4 (Table 2). The water absorption index for
grain may be a measure of steeping performance. Steeping is the first critical
step to ensure a clean separation of germ, endosperm, and pericarp during milling.
Hsu et al. (1983) found a negative correlation between absorption rate
and kernel size. The structure of the starchy endosperm was also found to be
very significant in affecting the rate of moisture penetration, with the subaleurone
region appearing to be rate limiting. Stenvert and Kingswood (1977) observed
that the more ordered the endosperm structure, the slower the rate of moisture
movement. Protein content and distribution were also of significant importance
because they generally contributed to a more ordered endosperm structure. The
rate of water penetration appears to relate to either grain hardness or protein
content.
There were significant differences in kernel size (dimensions)
and kernel components (Table 3). Kernel length ranged from 10.1 to 12.8 mm,
width 8.13 to 9.03 mm and thickness 3.47 to 4.10 mm. These values fall within
a range typical of maize cultivars as reported by Kirleis and Stroshine (1990).
Among the Hybrids, 8338-1 had the highest proportion of endosperm and 8705-4
had the lowest. Hybrid 8505-5, which had intermediate percent endosperm and
pericarp, had a signifi-cantly higher percent germ than any other hybrid. Hybrid
8338-1 had the lowest percent germ and intermediate percent pericarp. Percent
pericarp was highest for 8705-4 and lowest for 8522-2.
Table 3. Means for kernel size (dimensions)
and kernel components of maize hybridsa |
Hybrid |
Length (mm) |
Width (mm) |
Thickness (mm) |
Endosperm (%) |
Germ (%) |
Pericarp (%) |
8321-18 |
12.00cd |
8.17c |
3.47d |
79.9bcd |
13.7bcd |
5.66ab |
8321-21 |
12.17bc |
8.13c |
3.70cd |
80.7abc |
13.7bcd |
4.88cd |
8338-1 |
12.77a |
8.63ab |
3.57cd |
81.8a |
12.4e |
5.08bcd |
8425-8 |
10.13h |
8.43bc |
4.00ab |
81.1ab |
12.6de |
5.63abc |
8505-5 |
12.60ab |
8.53bc |
3.57cd |
79.1cd |
15.0a |
5.23bcd |
8522-2 |
10.87g |
8.50bc |
4.00ab |
81.4ab |
13.5cde |
4.49d |
8644-27 |
11.53ef |
9.03a |
4.10a |
79.3cd |
14.6abc |
5.55abc |
8644-31 |
11.57def |
8.73ab |
3.80bc |
80.4abc |
13.6bcde |
5.31bc |
8644-32 |
11.17fg |
8.80ab |
4.03ab |
79.2cd |
14.7abc |
5.56abc |
8705-4 |
11.70de |
9.03a |
3.47d |
78.3d |
14.7ab |
6.20a |
8705-6 |
12.57ab |
8.53bc |
3.83abc |
79.1cd |
14.3abc |
5.17bcd |
Mean |
11.73 |
8.59 |
3.77 |
80.0 |
13.9 |
5.34 |
LSD (0.05) |
0.45 |
0.41 |
0.29 |
1.72 |
1.23 |
0.75 |
C.V.(%) |
2.26 |
2.83 |
4.59 |
1.27 |
5.21 |
8.34 |
aMeans within a column followed by the same letter
are not significantly different (P<0.05) |
Chemical properties. Hybrids exhibited signi-ficant
variation for protein, fat, ash, crude fibre, and total sugars (Table 4). Hybrid
8644-31 had higher percent protein, ash and crude fibre but was low in percent
total sugars. Protein content ranged from 9.45 to 11.40, which is within the
common range for maize. Hybrid 8705-6 had the highest fat content while 8338-1
had the lowest. Normal maize grain contains between 3.5 - 5% fat. In the present
study, most of the hybrids could be considered high for fat content, except
8522-2 and 8338-1. Although the lipid content of maize is controlled to a large
extent by genetic factors (Watson, 1987), we cannot determine whether the high
fat content observed in this study was the result of genetic or environmental
effects, because the maize was grown in a single environment.
Table 4. Means for percent protein, fat, ash,
crude fiber and total sugars of maize hybridsa |
Hybrid |
Protein (%) |
Fat (%) |
Ash (%) |
Crude fiber (%) |
Total Sugars (%) |
8321-18 |
10.40cd |
8.45cd |
2.80c |
3.15c |
5.69a |
8321-21 |
10.05de |
7.40e |
2.15e |
2.35d |
3.66d |
8338-1 |
9.85ef |
3.35g |
1.50f |
1.40e |
3.04f |
8425-8 |
9.55fg |
8.05d |
2.50d |
2.55d |
3.54de |
8505-5 |
11.00b |
8.00d |
3.30ab |
3.75a |
4.30c |
8522-2 |
9.45g |
4.70f |
1.40f |
1.55e |
3.17ef |
8644-27 |
10.15cde |
8.80bc |
3.20b |
3.60ab |
5.19b |
8644-31 |
11.40a |
8.40cd |
3.45a |
3.60ab |
3.04f |
8644-32 |
11.30ab |
7.30e |
3.15b |
3.40b |
4.94b |
8705-4 |
11.05ab |
9.05b |
2.80c |
3.00c |
3.91cd |
8705-6 |
10.45c |
9.80a |
2.55d |
2.35d |
3.91cd |
Mean |
10.42 |
7.57 |
2.61 |
2.79 |
4.04 |
LSD (0.05) |
0.37 |
0.50 |
0.21 |
0.24 |
0.46 |
C.V.(%) |
1.62 |
2.98 |
3.64 |
3.89 |
5.21 |
aMeans within a column followed by the same letter
are not significantly different (P<0.05) |
When oil is not a desired end product, low fat content in maize
grain is preferred for most industrial and food-uses. High fat content in flour
or grits causes rancidity during storage and interferes with efficiency and
performance during secondary processing such as in the brewing industry. Even
with traditional processing, high fat content limits the shelf life of whole
maize meal, flour, and other products. Nonetheless, maize oil may be desirable
in some situations as a source of the essential fatty acid (linoleic acid) in
the diet. Fats contribute approximately 2.25 times more metabolisable energy
(ME) than starch or protein on an equal-weight basis. The fat in maize contributes
about 10-12% of the total ME provided by the maize (Wright, 1987). Therefore,
maize with a higher amount of fat would be beneficial to the animal feed industry.
Differences in ash and crude fibre content among the eleven
hybrids are presented in Table 4. Hybrids 8644-31 and 8505-5 had the highest
values for both ash and crude fibre content, while 8522-2 had the lowest values.
Peplinski et al. (1989) reported values of ash between 1.3 and 1.5% and
a fibre content of 2% for six maize hybrids. Our results show a range of 1.4-3.3%
for ash and 1.40-3.75% for crude fibre content. Total sugars ranged from 3.04
to 5.69%. Martin et al. (1976) reported a value of 7.4% total sugars
for dent maize. Hybrid 8321-18 had the highest content of total sugars while
hybrid 8338-1 had the lowest.
There were significant differences among hybrids for percent
starch, amylose, amylopectin content and water binding capacity of starch (Table
5). Hybrid 8338-1 was high in starch content while 8644-27 was low in starch.
Starch is the most important carbohydrate consumed on a world-wide basis because
it is continuously available in abundant quantities at low cost. Amylose, which
makes up 25-30% of maize starch, is essentially a linear molecule of glucose
units while amylopectin, constituting 70-75% of normal maize starch, is a branched
molecule. Although amylose content was significantly higher in Hybrid 8321-18,
it was still within the normal range (24-28%) reported for maize. Hybrid 8321-21
had the lowest amylose content. Hybrid 8321-21 had higher amylopectin content
but was low for amylose content.
Table 5. Means for percent starch, amylose,
amylopectin content and water binding capacity of starch of maize hybridsa |
Hybrid |
Starch (%) |
Amylose (%) |
Amylopectin (%) |
Water binding capacity (%) |
8321-18 |
69.41hi |
28.50a |
71.50e |
162.6b |
8321-21 |
74.24c |
21.50e |
78.50a |
150.1cd |
8644-27 |
69.01i |
26.00c |
74.00c |
177.0a |
8644-31 |
70.06fg |
26.50c |
73.50c |
159.2b |
8644-32 |
69.96fg |
26.00c |
74.00c |
156.3bc |
8522-2 |
79.63b |
27.55b |
72.45d |
150.8cd |
8425-8 |
73.66d |
24.75d |
75.25b |
145.0d |
8705-4 |
70.09f |
26.75c |
73.25c |
150.0cd |
8705-6 |
70.79e |
26.55c |
73.45c |
145.8d |
8338-1 |
80.76a |
28.00ab |
72.00de |
156.1bc |
8505-5 |
69.60gh |
24.90d |
75.10b |
149.1d |
Mean |
72.47 |
26.09 |
73.90 |
154.73 |
LSD (0.05) |
0.46 |
0.75 |
0.75 |
6.72 |
C.V. |
0.29 |
1.31 |
0.46 |
2.57 |
aMeans within a column followed by the same letter
are not significantly different (P<0.05) |
The relative proportions of amylose and amylopectin greatly
influence the physico-chemical properties of starch, and, therefore, its technological
and nutritional properties. Although it has been indicated that higher amylose
content is associated with lower starch digestibility (Martinez and Lausanne,
1996), it is not likely that the observed differences in amylose content would
have any effect on starch digestibility. The amylose content of flour is a key
parameter in the food industry since it can strongly influence the physical
and chemical properties of flour, such as viscosity, retrogradation, solubility
or water absorption (Martinez and Lausanne, 1996), as well as the bioavailability
of starch and its interaction with other food components. Sterling (1978) noted
that gelatinisation temperature seems to depend on the relative proportions
of amylose and amylopectin, such that higher amylose contents are associated
with higher gelatinisation temperatures.
The water binding capacity of starch ranged from 145 to 177%
(Table 5). Water binding capacity of Hybrid 8644-27 was relatively high and
differed significantly from other hybrids. Inherent differences in the proportion
of crystalline and amorphous areas in starch granules may contribute to differences
in water binding capacity, with presumably greater water binding capacity in
starch containing large proportions of amorphous area. Medcalf and Gilles (1965)
reported that, in general, the higher amylose starches have higher water binding
capacity.
CONCLUSIONS
A wide range of differences in physical, chemical, and water
absorption capacity exist among the eleven maize hybrids investigated. None
of the hybrids excelled in all quality criteria. For example, Hybrid 8644-31
was relatively high for test weight, thousand kernel weight, hardness index,
percent protein, ash, and crude fibre content. However, it also exhibited low
starch content, total sugars, and water absorption index of grain. Hybrids with
low percentage floaters took longer to grind as indicated by the hardness index,
which may reflect a greater proportion of corneous endosperm. Hybrid 8321-18
was an exception, having zero percent floaters, but relatively low hardness
index.
The presence of genetic variation among the hybrids suggests
that a potential exists for improvement of grain quality to suit specific processing
and food use requirements through selection and breeding. Breeding programmes
in developing countries should target hybrid development to meet the requirements
of producers, processors, and consumers. Hybrids with suitable quality characteristics
for preferred food preparations and industrial uses could provide markets for
the private seed sector, thereby promoting both availability of high quality
seed for farmers and increased demand for maize products.
ACKNOWLEDGEMENTS
Financial support for this work was provided solely by Guinness
Breweries, Nigeria, Ltd. and is highly appreciated. We are grateful for the
assistance of N.S. Ilo, A.F. Adedapo, and T. Olatunji in carrying out the grain
quality analyses.
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